This invention relates to a thermal transfer printer consisting of a guide for the object medium to be imprinted as well as a print head and a print unit incorporating a transport assembly for the thermal transfer tape.
In a thermal transfer printer, an object medium is imprinted by means of a thermal transfer tape which, during the printing process, travels past the print head in synchronism with the feed rate of the object medium to be imprinted. The thermal transfer tape consists of a substrate on which at least one ink or dye is deposited. During the printing process the print head heats up individual points of the thermal transfer tape that travels past it, corresponding to the image to be imprinted on the object medium, causing dye particles to be fused, separated from the substrate and adhesively transferred to the object medium.
For the flawless imprinting of an object medium it is necessary for both the medium and the thermal transfer tape to move past the print head at precisely the same speed. In prior art, the feed rate of the object medium and the speed of travel of the thermal transfer tape are synchronized by various control elements. The drawback in this case, however, is the typically complex design of these elements with attendant susceptibility to breakdown, and in the event of a malfunction any corrective action usually tends to be quite expensive in terms of time and money.
Given the said prior art, it is the objective of this invention to provide a thermal transfer printer which allows for simple synchronization of the thermal-transfer tape speed and the feed rate of the object medium, and which displays little susceptibility to breakdowns.
The technical solution offered by this invention is a thermal transfer printer which is characterized in that it incorporates a direct, positive drive for the thermal transfer tape, moving the thermal transfer tape past the print head at a speed that matches the feed rate of the object medium to be imprinted.
The thermal transfer printer offered by this invention permits considerably improved synchronization of the thermal-transfer tape speed and the feed rate of the object medium to be imprinted.
The thermal transfer printer according to this invention incorporates a transport mechanism for the thermal transfer tape which consists essentially of a drive unit, a supply spool, a pickup spool and guide rollers. For printing on an object medium, fresh, unused thermal transfer tape is unreeled off the supply spool, guided past the print head by means of suitable guide rollers and, upon imprinting the object medium, wound onto the pickup spool as spent thermal transfer tape. Most desirably, both the pickup spool and the supply spool are in the form of cylindrical drums exchangeably mounted in the thermal transfer printer. The drive unit for transporting the thermal transfer tape consists of a motor and at least one motor-driven drive cylinder. In operation, the drive unit moves the thermal transfer tape past the print head at a tape speed that matches the feed rate of the object medium to be imprinted.
According to another desirable feature of the invention, the drive cylinder is in direct contact with the circumference of the cylindrical pickup spool, driving the latter at a constant circumferential speed independent of the diameter of the tape coil, as a result of which the thermal transfer tape will always travel past the print head at a linearly constant tape speed.
As another desirable feature offered by this invention, the pickup spool is displaceable relative to the drive cylinder so that, even as the diameter of the coiled tape increases, the positive drive action by the drive cylinder that is in contact with the circumference of the pickup spool is maintained. This is accomplished by suspending the exchangeable pickup spool in elongated slots.
As another desirable feature according to this invention, the drive cylinder is provided with a heating element which heats up as the thermal transfer tape is wound onto the cylindrical pickup spool. Energizing the heating element causes the thermal transfer tape traveling past the drive cylinder to be briefly heated up before being wound onto the cylindrical pickup spool, as a result of which the individual layers of the thermal transfer tape on the pickup spool adhere to one another. The mutually adhering layers of the thermal transfer tape wound on the pickup spool can no longer slip and slide relative to one another, the desirable result being a tight coiling of the thermal transfer tape onto the cylindrical pickup spool. The preferred heating element is a resistor and preferably a PTC resistor. As another desirable, proposed feature of this invention, the heating element is rod-shaped and protrudes into a circular groove in the drive cylinder.
As another desirable feature offered by this invention, the guiding of the thermal transfer tape is aided by an additional guide roller which is preferably in the form of a tapered, cylindrical centering roller serving to center the thermal transfer tape on the pickup spool. The centering roller is preferably suspended in an elongated slot which allows it to be retracted upwards into a standby position, thus facilitating the loading of the thermal transfer tape.
According to another advantageous aspect of this invention, the cylindrical pickup spool incorporates a braking mechanism as well as a free-wheeling capability. The braking mechanism assures both a defined, dependable protection against unraveling and a short rewind path. In desirable fashion, both features help prevent any breaking of the thermal transfer tape and permanently keep the thermal transfer tape sufficiently taut. The free-wheeling capability advantageously permits manual rewinding of the thermal transfer tape.
As another desirable feature of the invention, the guide system for the object medium to be imprinted is positioned opposite the print head at a distance that defines a working area, the said guide system incorporating a transport as well as an adjustment device by which the distance between the guide system and the print head can be varied. The guide system serving to channel the object medium is designed to allow the print head and the object medium to be brought together in desirable fashion, whereby it is not the print head that is moved toward a positionally fixed object medium but, instead, the guide system moves the object medium toward the print head into printing position.
As another desirable feature offered by the invention, the transport consists of a transport belt and a belt drive, the transport belt guided by at least two guide rollers which are positioned in a plane essentially parallel to the print unit. Most desirably, the transport belt forms a continuous loop and is driven by a stepping motor via the guide rollers. The transport belt carries an object medium, sheet-fed into the thermal transfer printer, past the print unit along a plane essentially parallel to that of the print unit, with the space between the object medium transported by the belt and the print unit being adjustable. The medium-accommodating working space created by mounting the transport at a distance from the print head makes it possible in desirable fashion to feed in object media of various thicknesses.
As another advantageous feature of the invention, the guide system encompasses an adjustment capability consisting of a drive and a motion transfer assembly, with at least part of the motion transfer assembly positioned within the continuous loop of the transport belt and movable in relation to the latter. The motion transfer assembly is positioned opposite the print unit at a distance that defines a working space, with a displacement of the motion transfer assembly in a direction essentially perpendicular to the print unit permitting the distance between the object medium and the print unit to be augmented and reduced as required.
After an object medium to be imprinted is introduced in the working space between the print unit and the guide system, it is fixed in its position and, by a displacement of the motion transfer assembly toward the print unit, it is moved up to the print unit that contains the print head. Thus, the thermal transfer printer according to this invention makes it possible to bring the print unit and the object medium to be imprinted together regardless of the thickness of the object medium. The motion transfer assembly and the print unit face each other, which results in a straight, linear feed movement of the motion transfer assembly. As a consequence, even when object media of various thicknesses are fed in, no further adjustment of the motion transfer assembly is necessary.
As another advantageous feature of this invention, both the linear tape speed of the thermal transfer tape traveling past the print head and the linear feed rate of the transport belt carrying the object medium are adjustable. As a particularly desirable feature, a control unit is provided which synchronizes the tape speed of the thermal transfer tape and the feed rate of the transport belt.
For the thermal transfer printer according to this invention, a positive-drive unit is proposed which acts directly on the thermal transfer tape and which in desirable fashion moves the thermal transfer tape past the print head at a linearly constant tape speed. The drive unit is preferably a motor and at least one drive cylinder which latter is in direct contact with the circumference of the cylindrical pickup spool, driving the spool. With this configuration of drive cylinder and pickup spool the pickup spool will always be driven at the same circumferential speed and the tape speed will always remain constant regardless of the diameter of the coiled tape on the spool. The speed of the thermal transfer tape is preferably selectable. The guide system of the thermal transfer printer, serving to feed in the object medium, preferably consists of a transport and an adjustment device, with the transport and the adjustment device coupled to each other, thus obviating the need for an additional transport and adjustment synchronizing provision for feeding the object medium to the print unit. The guide system and the print unit face each other at a distance that defines a working space between them, whereby an object medium introduced in the thermal transfer printer is moved past the print head by means of a transport belt in a plane essentially parallel to the print head. In desirable fashion, the transport belt is driven by a stepping motor in a manner that the object medium carried by the transport belt is fed to the print head at a linearly constant speed. The feed rate of the object medium carried by the transport belt is preferably adjustable. Due to the fact that both the thermal-transfer tape speed and the feed rate of the object medium are linearly constant while the said tape speed and feed rate are nevertheless adjustable, the thermal transfer tape can travel past the print head in synchronism with the object medium.
For printing on the object medium, the motion transfer assembly is shifted toward the print unit containing the print head, pushing the object medium against the latter. During that process the object medium is continually carried forward by the transport belt, thus permitting a continuous printing operation. Upon completion of the printing process the motion transfer assembly returns to its home position and the imprinted object medium is released from the print unit. In advantageous fashion, the guide system makes it possible for an object medium to move past the print unit at a certain distance without being pressed against the print unit by the motion transfer assembly.
Thus, in desirable fashion, the thermal transfer printer according to this invention assures synchronous travel of both the thermal transfer tape and the object medium past the print head as well as the synchronization of the transport and the adjustment device without requiring costly and complex control elements, making the thermal transfer printer according to this invention highly fail-safe and easy to maintain.
When the motion transfer assembly is displaced toward its lower end position in the direction opposite that of the print unit, a guide serving to center the thermal transfer tape is likewise moved downwards by virtue of its positive connection with the motion transfer assembly, providing an opening for the replacement of the thermal transfer tape.